Deprecate the unstable concat_idents!

`concat_idents` has been around unstably for a long time, but there is
now a better (but still unstable) way to join identifiers using
`${concat(...)}` syntax with `macro_metavar_expr_concat`. This resolves
a lot of the problems with `concat_idents` and is on a better track
toward stabilization, so there is no need to keep both versions around.
`concat_idents!` still has a lot of use in the ecosystem so deprecate it
before removing, as discussed in [1].

Link: rust-lang#124225
[1]: https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/Removing.20.60concat_idents.60

Author: tgross35

core/src/macros/mod.rs

@@ -1138,6 +1138,10 @@ pub(crate) mod builtin {
         issue = "29599",
         reason = "`concat_idents` is not stable enough for use and is subject to change"
     )]
+    #[deprecated(
+        since = "1.88.0",
+        note = "use `${concat(...)}` with the `macro_metavar_expr_concat` feature instead"
+    )]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! concat_idents {

core/src/prelude/v1.rs

@@ -59,6 +59,7 @@ pub use crate::hash::macros::Hash;
 
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 #[allow(deprecated)]
+#[cfg_attr(bootstrap, allow(deprecated_in_future))]
 #[doc(no_inline)]
 pub use crate::{
     assert, cfg, column, compile_error, concat, concat_idents, env, file, format_args,

std/src/lib.rs

@@ -709,6 +709,7 @@ pub use core::primitive;
 // Re-export built-in macros defined through core.
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 #[allow(deprecated)]
+#[cfg_attr(bootstrap, allow(deprecated_in_future))]
 pub use core::{
     assert, assert_matches, cfg, column, compile_error, concat, concat_idents, const_format_args,
     env, file, format_args, format_args_nl, include, include_bytes, include_str, line, log_syntax,

std/src/prelude/v1.rs

@@ -46,6 +46,7 @@ pub use crate::result::Result::{self, Err, Ok};
 // Re-exported built-in macros
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 #[allow(deprecated)]
+#[cfg_attr(bootstrap, allow(deprecated_in_future))]
 #[doc(no_inline)]
 pub use core::prelude::v1::{
     assert, cfg, column, compile_error, concat, concat_idents, env, file, format_args,